Commercially available wet process phosphoric acids are generally manufactured from either calcined rock or uncalcined rock. Calcining decomposes and drives off the organic matter in the rock, and the phosphoric acid product made by dissolving it, known as green acid, contains almost no suspended organic solids. When uncalcined rock is digested, considerable amounts of organic compounds are dissolved from the phosphate rock and remain as both soluble and insoluble impurities in the product acid, known as "black" or "brown" acid. The organic compounds in the acid are commonly referred to as humic acids. Uranium and other metals can be recovered from this commerical grade wet process phosphoric acid. Such recovery processes, directed primarily to uranium, are taught by Hurst and Crouse, in U.S. Pat. Nos. 3,711,591 and 3,835,214, relating to reductive stripping and oxidative stripping respectively.
The main purpose in mining phosphate rock has been to produce fertilizer. As an initial step in making fertilizer, the concentrated and milled phosphate rock is reacted with sulfuric acid under constant and intensive agitation, to produce a phosphoric acid solution and insoluble calcium sulfate (gypsum). A simple form of the reaction is expressed as follows: EQU 2Ca.sub.3 (PO.sub.4).sub.2 +6H.sub.2 SO.sub.4 +12H.sub.2 O=4H.sub.3 PO.sub.4 +6CaSO.sub.4.2H.sub.2 O
The wet process phosphoric acid solution, formed as shown above, generally contains about 600 grams/liter of H.sub.3 PO.sub.4, about 0.1 to 0.2 gram/liter of uranium, and substantial amounts of organic humic acid impurities. This metal containing acidic solution can be processed to remove the valuable uranium. For example, the solution of phosphoric acid can serve as the aqueous feed in a liquid-liquid solvent extraction process of uranium recovery.
To make the metal recovery process viable, however, it is necessary that the acidic solution be highly purified, in order to control sludge emulsion formation in the solvent extraction mixer-settlers used in the metal recovery process. This sludge problem, caused by the humic acid impurities, was recognized by Hurst and Crouse in U.S. Pat. No. 3,711,591.
Reese et al., in U.S. Pat. No. 4,087,512, attempted to solve problems of uranium extraction emulsions and sludge formation, caused by organic humic acids in the wet process phosphoric acid feed, by a pretreatment with a hydrocarbon at from 55.degree. C. to 70.degree. C. The volume ratio of acid:hydrocarbon was as high as 30:1 and was preferably 2:1. The useful hydrocarbons used by Reese et al. including kerosene, gasoline, benzene and toluene. After the acid and the hydrocarbon are mixed, they are transferred to a separator with a conical bottom. After 5 to 10 minutes, the mixture separates into hydrocarbon phase, composed of hydrocarbon, emulsified solid organic materials and some captured phosphoric acid, and a purified heavier bottom aqueous phase, composed of phosphoric acid. The bottom aqueous phase is drawn from the conical bottom of the separator. The entire top hydrocarbon phase overflows the top of the separator into a settling chamber for further separation. Such a process, involving interaction of wet process phosphoric acid solely with a kerosene type hydrocarbon has not been found effective in removing a major portion of the organic humic acid over an extensive time period.
Simpler mechanical removal methods, such as pumping at the interface, after allowing sludge emulsions to form, are not particularly effective, because large quantities of expensive solvent are also pumped out. This pumping method is also hampered by the non-Newtonian flow properties of the sludge emulsion, which cause it to deviate from a normal pipe entrance behavior.
Various flotation processes have been used to concentrate valuable mineral particles, such as copper, molybdenum, cobalt and nickel particles from sulfide, phosphate, fluorite, and chromite ores in the mining industry, as described in Flotation Fundaments, Dow Chemical Co., Sections 1-3, 1970. Generally, ore feed is finely ground to release the valuable mineral particles. A hydrophobic film is formed on the valuable mineral particles to be removed by flotation, and a hydrophilic or wettable film is formed on the residue impurities by addition of various collecting, modifying and frothing agents. These additions allow adherence of the valuable mineral particles to air bubbles in a froth flotation process to provide a recoverable mineral laden froth on the surface of the ore pulp. Useful hydrophobic film forming collecting agents include heteropolar, i.e., containing both charged and uncharged groups, cationic materials, such as alkyl amines, and quaternary ammonium compounds, and anionic materials, such as xanthates, thionocarbamates, dithiophosphates, thiocarbanilide, xanthogen formates, fatty acids, and sulfonates. Various flocculants are separately discussed as providing high filter and improved cyclone separations for mill operations in the mining industry to improve the efficiency of solids-liquid separations. Useful flocculants include non-ionic, anionic and cationic high molecular weight, water soluble polymers.
What is needed is a pretreatment process to purify acidic solutions, by removing substantially all of the organic humic acids, which form sludge or emulsions at the phase interfaces during solvent extraction in metal recovery processes. The pretreatment process must be low in capital cost, and should result in low operating costs and operator attention.